Field of the Invention
The present invention relates to simple transmissions utilizing compound countershafts and, in particular, relates to simple transmissions of the type utilizing a multiplicity of substantially identical compound countershafts and a rotatable mainshaft which is arranged substantially parallel to the countershafts with means provided for pivotally supporting the output end of the mainshaft and means provided for guiding the input end of the mainshaft in free floating relation relative to the axis of the countershafts and wherein the mainshaft gear closest the pivot point of the mainshaft is radially and axially fixed relative to the mainshaft while all or substantially all of the remaining mainshaft gears are located generally concentric of the mainshaft and in constant meshing engagement with and supported solely by the countershaft gears and wherein the countershafts are each comprised of portions which are in constant meshing engagement through an idler gear assembly which is concentric with but freely floating relative to the mainshaft.
Simple transmissions, that is, transmissions utilizing a one piece mainshaft, are well known in the prior art. Transmissions utilizing either simple or compound mainshafts in connection with multiple countershafts wherein either the mainshaft gears or the countershaft gears are mounted in a radially floating manner relative to the other gears are well known in the prior art. Examples of such transmissions may be seen by reference to U.S. Pat. Nos. 3,105,395; 3,255,644; 3,283,613; 3,335,616; 3,349,635; 3,378,214; 3,500,695; and 3,648,546.
The prior art transmissions, particularly those transmissions utilizing a floating mainshaft and/or floating mainshaft gears in connection with multiple countershafts, have proven to be highly acceptable. However, to achieve a sufficiently large range of available gear ratios, it has generally been necessary to utilize a compound transmission. Such transmissions generally comprise a main transmission and an auxiliary transmission of either the "range" type or the "splitter" type or a combination thereof as is well known in the prior art. Such transmissions are highly effective and commercially successful, especially for use in heavy duty vehicles wherein a large range of ratios is required and a large number of individual gear ratios is required. Such heavy duty compound transmissions typically have 9, 10, 12, 13 or more forward gear ratios.
There has, however, developed a need for transmissions having a greater range of ratios and a greater number of ratios than is normally available in a simple transmission as the centerline distances required in prior art simple transmissions to provide same would become too great while not justifying the somewhat expensive structure and controls required for a compound transmission. The prior art devices have been unable to satisfactorily fill this need.
Simple transmissions utilizing multiple compound countershafts have been proposed wherein all mainshaft gearing was floating relative to the mainshaft and wherein the idler gear assembly was substantially rigidly mounted to the mainshaft to prevent the countershaft portions most distant the input gear from counter rotating at rest conditions which would tend to cause the mainshaft gears to sag or bottom out on the mainshaft which in turn would result in misalignment of the clutches and subsequent shifting difficulties. This was not a totally satisfactory design as the idler assembly defined a substantially rigid coupling between the front and rear portions of the countershafts and tended to transmit timing or indexing errors from the front countershaft portions to the rear countershaft portions.